Idealized Carbon Nanotubes (CNT) can be visualized as 3-D graphite sheets rolled to form seamless cylinders, closed with end caps on both the sides. These end caps have fullerene like structure. A defect free nanotube has exceptional mechanical, electrical and thermal properties. The defects can be in the structure or in the morphology. These structures have stability greater than that of graphite, thermally as well as chemically. Based on the diameter of the nanotubes, CNTs are broadly classified into: (1) Single Wall Carbon Nanotubes; (2) Multi Wall Carbon Nanotubes; and (3) Carbon Nanofibers.
Since carbon nanotubes have such extraordinary mechanical properties, one would expect that, when these are embedded into some other matrix material, the properties of the matrix would improve drastically. But, this actually does not happen. This is because of poor load transfer between the nanotubes and matrix. Nanotubes are chemically very inert and are not compatible with any other material in their pristine state. Also, because nanotubes are typically not well-dispersed, they create stress concentrations and the high surface area of the nanotubes is compromised. To fabricate composites with improved properties, it is important to modify the surface of the nanotubes so as to achieve a good interface and at the same time, improve the dispersion of the carbon nanotubes in the matrix. The poor dispersion and poor interfacial interactions between the nanotubes and the matrix also result in lower than expected electrical and thermal properties in the resulting composite material.
To more optimally fabricate such composites, various modifications have been proposed. Instead of dispersing the nanotubes in the matrix and then using the matrix to make composite structures, an attempt is made to grow the nanotubes on the composite preforms and then fill the preforms with matrix material. Using this procedure, the problem of dispersion of nanotubes can be by-passed.
The present invention is useful for applications in numerous industries including composite materials, filtration materials, electrodes, membranes, cell growth supports, catalysis, and many other novel and emerging applications that will benefit from this unique technology. In particular, the present invention relates to novel non-woven, woven and braided continuous fiber composite preforms that are subsequently reinforced with vapor grown carbon fibers that are grown in situ in the preform using a continuous growth process, the preforms so made and composite articles made using the preform. The resulting continuous fiber VGCNT reinforced composite preform exhibits increased fiber volume fraction of reinforcing fibers and greatly increased surface area thus improving the strength, stiffness, electrical conductivity, and thermal conductivity of polymer matrix composites produced from these preforms while maintaining the manufacturing benefits of a continuous non-woven, braided or woven preforms. The resulting articles produced from VGCNT infused preforms produced in this way are useful for numerous applications that take advantage of the unique structural, morphological, electrical, and thermal properties.
Polymer matrix composites are well known for use in structural and thermal-structural applications. Continuous yarn, and other multidimensional 2-D and 3-D, non-woven, woven or braided, composite preforms are used in the manufacture of reinforced composites due to their economical manufacturing processes. For the purposes of this invention the term “preform” means a continuous fiber yarn, tow, or broad good produced from the tow or yarn (including non-woven mats, woven or braided constructions) and assemblies of preforms further constructed. Through weaving or braiding of the reinforcing fiber yarns of carbon or graphite (carbon and graphite fibers are generally referred to collectively as “carbon fiber” and the term “carbon fiber” is used throughout to mean “carbon and/or graphite fiber”), glass, quartz, metal or ceramic fiber a composite “preform” can be manufactured into a near net shape that is subsequently infused with a polymer resin and cured in a mold to manufacture articles.
Preforms may also be infused with a polymer or polymer resin to manufacture a prepreg useful for the fabrication of polymer matrix composite material articles. Using well-known methods the non-woven, woven or braided preforms are manipulated by slitting, combining together, stitching together, shaping, or other methods to assemble a near net shape preform for the fabrication of a composite article. The continuous fiber preform processes are advantageous since they can be used very economically to produce a variety of shapes useful in the manufacture of composites. A fundamental limitation of certain woven, braided, non-woven mat, or felt preform technology to date, however, has been that the resulting composites manufactured from these preforms are of lower strength and stiffness than composites manufactured using other methods. This is due in part to the lower fiber volume fraction that results from the weaving and braiding processes and in part to the failure mechanisms of weave braid or tow composites intrinsically related to the reinforcing fiber geometry and architecture.
A goal in composite materials design has been to obtain materials which exhibit high stiffness, strength, fracture toughness, controllable electrical and thermal properties and can be affordably manufactured. Hence, there exists a need for a novel approach to improve the stiffness, strength, fracture toughness, and the electrical and thermal properties, of woven and braided composite materials while maintaining the low cost advantages of continuous woven and braided preform manufacturing.
The present invention is concerned with the use of VGCNT produced in a composite preform to improve the mechanical, electrical, and thermal characteristics of composite materials produced from these preforms as well as novel materials and articles that can be produced from the preforms themselves. VGCF and VGCNT are produced directly from hydrocarbons such as methane, acetylene, methane, propane, ethane, ethylene, benzene, natural gas or hydrocarbon gas mixture, in a gas phase reaction upon contact with a catalytic metal particle in a non-oxidizing gas stream. Various reaction processes, conditions, and chambers are known and described in e.g., U.S. Pat. Nos. 5,024,818 and 5,374,415 for the manufacture of VGCF. Vapor grown carbon fibers differ substantially from commercial carbon fibers in that the VGCF are not continuous. The VGCF and VGCNT can vary in diameter and length depending on processing parameters, including catalyst particle characteristics, reactive gas composition, pyrolysis time and temperature, heat treatment time and temperature, and length of growth period and volume of furnace, but exhibit diameters in the range of 1 to 500 nm and lengths in the range of 0.1 μm to 500 μm.
More importantly and pertinent to this application is that the fiber diameter of a vapor grown carbon fiber is generally under 1 μm. As those familiar with the growth of vapor grown carbon fibers know, these fibers can be subsequently thickened to the diameter of commercial fibers. However, these fibers are not as desirable from an economic or performance perspective. It is desirable to use fibers that are smaller than the diameter of a commercial fiber by a factor in the range of 10 to 100.
Further, as the vapor grown carbon fibers are much finer than continuously produced carbon fibers they can be used effectively to increase the fiber volume fraction of a continuous fiber composite by occupying the void spaces between the continuous fibers. The fine diameter vapor grown carbon fibers can occupy spaces in non-woven, woven or braided composite preforms without perturbing the geometry, orientation, or continuous fiber volume fraction of the preform. The result is that the overall fiber volume fraction is increased leading to desirable changes in the mechanical, electrical, and thermal behavior of composite materials manufactured from these preforms.
A further distinctly novel advantage of this approach is that vapor grown carbon nanotubes are intimately and uniformly incorporated into a composite material. The in situ process to produce the nanotube reinforced preform ensures that nanofibers are well distributed throughout the preform and are in intimate contact with themselves and the continuous fiber of the preform. Conventionally, carbon nanotubes are grown, separated from their substrate and incorporated into a composite material by mixing and/or dispersing the nanofibers into the matrix. The nanofiber/matrix mixture is then used to prepare composites. This method has many disadvantages including cost, additional process operations, nanofiber damage from mixing, and negative impact on matrix rheology. Further, the nanofibers must be handled and possible health risks from nanofiber exposure is a concern. U.S. patent application Ser. No. 11/057,462 discloses methods in which VGCF are produced directly from catalytic particles formed on the continuous fiber surface, thus the nanofibers are fused to the continuous fiber surface and act to enhance the adhesion of the composite matrix to the continuous fiber, further improving the properties of resulting composite materials. However, the invention described in Ser. No. 11/057,462 is limited in that the continuous preform is static in the reaction vessel and is therefore limited in its length to the size of the vessel and further, the reaction conditions to which the preform is subjected.
A further distinctly novel advantage of the present invention is to change the electrical conductivity of a non-conducting composite material preform at very low levels of nanotube. It is known that carbon nanotubes can be incorporated into a polymer matrix by mixing, blending, solvent-assisted blending, or other similar techniques. At a certain fraction of nanotubes, the polymer composites made in this way become conductive due to continuous contact of the inherently conductive nanotubes. The point at which this continuous conduction occurs is commonly referred to as the “percolation threshold.” In conventional nanotube composites where the nanotubes are mixed into the matrix material this typically occurs at weight fractions of nanotube to polymer of 1% to 30% depending on the nanotube morphology, mixing techniques, and other variable factors. In the novel approach described in this invention conductivity occurs at nanotube levels approximately ten times lower. This phenomenon is because the nanotubes are not broken down in aspect ratio and their intimate contact with each other is not disrupted by mixing and dispersion processes.
There are limited literature reports of attempts to produce catalytically vapor grown carbon nanotubes on graphite, carbon, quartz, glass or metal substrates. However, the methods differ substantially from the method described in this invention and none report continuous in situ production of VGCNT on carbon fiber yarns, tows, non-woven, woven or braided preforms and are thus further limited in their utility due to the limited ability to incorporate such VGCNT into a composite article.
Hernadi et al. (1996) report on VGCF produced on graphite flakes using an iron catalyst and acetylene/nitrogen gas mixture. They treated the graphite flakes with iron acetate and then reduced under hydrogen at 1200° C. to produce metallic iron particles. VGCF were subsequently produced at 700° C. in a flowing acetylene/nitrogen gas at atmospheric pressure. The reported yield was extremely low at 3.4% with poor quality nanofibers. Yacaman et al. (1993) also reported VGCF produced on graphite flakes using an iron catalyst and acetylene/nitrogen gas mixture. They treated the graphite flakes with an iron oxalate solution and reduced the catalyst to metallic iron particles under hydrogen at 350° C. VGCF were subsequently produced at 700° C. in a flowing acetylene/nitrogen gas at atmospheric pressure for several hours. They reported nanofibers were produced with diameters in the range of 5.0 to 20 nanometers and lengths of around 50 micrometers, however, after 1 hour of growth graphitic structures were noted around few catalytic particles. Ivanov et al. (1995) reported production of VGCF on graphite flakes using an iron catalyst and acetylene/nitrogen gas mixture. They treated the graphite flakes with an iron oxalate solution followed by calcination at 500° C. followed by reduction with hydrogen at 500° C. for 8 hours. Under optimal conditions they reported VGCF with average diameter of 40-100 nanometers and average length of 50 micrometers and 50% amorphous carbon. Wang et al. (2002) reported VGCF produced on graphite foil by sputter coating with stainless steel (Fe:Cr:Ni—70:19:11) followed by hydrogen reduction at 660 C. VGCF were subsequently produced at 0.3 torr pressure using an acetylene/nitrogen mixture. Significantly, they reported that for a pure iron or nickel catalyst on graphite no VGCF were formed. Thostenson et al. (2002) used identical process conditions as Wang et al. (2002) for growth of VGCF on a carbon fiber. They reported a nanofiber growth layer region between 200-500 nm in thickness.
U.S. Pat. Nos. 5,165,909 and 6,235,674 to Tennent et al., discuss the possibility of producing carbon fibrils, fibril mats, furry fibers, furry plates, and branched fibrils by deposition of a metal-containing particle on the surface of a carbon or alumina fiber, plate, or fibril and subsequent chemical vapor catalytic growth of carbon fibrils on the substrate at temperatures in the range of 850° C. to 1200° C. This example requires deposition of a preformed catalyst particle onto a carbon substrate and furthermore no working examples were provided other than branched fibrils. However, the approach Tennent et al., was very limited because it requires a separate process to form catalytic particles and disperse them. Such dispersion is not possible with a multi-filament yarn of continuous macroscopic fiber or a woven or braided preform manufactured from a multi-filament yarn.
In all these cases no discussion or method exists for the production of VGCNT on continuous carbon fiber yarns and preforms both mono- and multi-filament with sufficient yield in an industrially practical process. Thostenson et al. (2002) is the only literature report of VGCF growth on a carbon fiber, but in that case they used a stainless steel sputter coated fiber and specifically mentioned that catalyst could only be deposited on the outermost surfaces of a fiber bundle, not the interior fibers—and the process required a lengthy hydrogen reduction step to form catalytic particles. Further, VGCF growth was performed under high vacuum. None of these process steps are amenable to practical, scalable, manufacturing of nanofiber reinforced preforms.
U.S. patent application Ser. No. 11/057,462 (now U.S. Pat. No. 7,338,684, hereby incorporated in its entirety for all purposes) describes the fabrication of continuous preform having in situ grown VGCF. However, while the '462 application describes the benefits of in situ growth of VGCF on continuous fibers, the methods provided are limited to “batchwise” processing. The current invention provides for the in situ growth of VGCNT on continuous fiber preforms and continuous processing of the continuous fiber such that the time, type of fiber and characteristics of the VGCNT can be tailored to the demands of the preform or ultimate composite made therefrom. This eliminates the processing steps for isolated carbon nanotubes reported in other carbon nanotube composite approaches and therefore greatly reduces risk of environmental release and exposure to carbon nanotubes. A further limitation of the art is that to be usable, the VGCNT laden preforms need to be made in large volumes such that there use is not limited to short pieces or sections able to fit in a single reaction vessel.